A level sensor for detecting a predetermined level is known from DE 3348119 C2. This sensor contains a probe in the form of a vibrating rod that protrudes into the container and is excited to vibrate by an electric excitation device. The frequency of the vibration depends on the level in the container. A vibratory system that is usually referred to as an oscillator is realized by amplifying and feeding back the electrically detected mechanical vibration signal to the excitation input.
Similar methods and devices are described, for example, in DE 19720519 C2, EP 0985916 A1 and EP 0985917 A1. In all the aforementioned publications, the described vibratory level sensor contains, in principle, an electromechanical system that is excited to continuously self-oscillate. Various options that differ with respect to the design of the mechanical vibratory element, the electromechanical converter system and the electrical design of the oscillator are known for realizing an oscillator of this type.
It is preferred to utilize at least two vibrating rods that are either arranged coaxially or parallel to one another as the mechanical vibratory element.
This vibrational mechanical system is usually driven electromechanically by a piezo-element that is excited with an AC voltage, where a piezo-element is also used for detecting and reconverting the mechanical vibration into electrical vibration. In this case, the driving device and the detecting device can be divided into two separate piezo-elements, into two separate regions of one piezo-element or both functions can be combined in one piezo-element.
If separate driving and detecting elements are provided, the entire electromechanical vibratory system could also be interpreted as an electric quadrupole network that transmits a voltage applied to the two input poles to the two output poles with an amplitude and phase shift that depend on frequency. The transmission characteristics of this quadrupole network are comparable to those of an electronic resonant circuit.
A single piezo-element that is used for driving and detecting purposes usually contains two connections. This is the reason such a piezo-element can also be interpreted as an electric dipole network. Electric dipole networks are generally characterized by their impedance, which may be dependent on frequency.
In this context, it should also be noted that a single piezo-element which can also be interpreted as an electric dipole network is able to simulate the characteristics of an electric quadrupole network if it operates in accordance with the time-division multiplex mode, i.e., if the driving and the detection function are periodically changed over at the connection poles.
Conventional electric oscillators can be realized with the described oscillating systems that should be interpreted as electric dipole networks or quadrupole networks. An oscillator always contains amplifying and feedback devices, of which at least one should be frequency-selective. Preferred embodiments of oscillators contain, for example, a frequency-selective quadrupole network that should be interpreted as an electronic resonant circuit in the feedback loop of the feedback amplifier circuit.
Other embodiments of oscillators are characterized by a feedback amplifier in which the amplification is dependent on the oscillatory system that should be interpreted as an electric dipole network. The amplification of the thereby formed frequency-selective amplifier is dependent on the impedance of the dipole network that characterizes the vibratory structure.
A problem with all these vibratory level sensors is that the oscillator must be designed such that a reliable stimulation of the oscillations and a continuous vibration of the oscillator are ensured under all operating conditions. For example, if the sensor is immersed in the material, the attenuation of the vibratory system is increased such that its natural frequency and phase shift change. This causes the feedback or amplification characteristic of the oscillator to be subjected to more less intense fluctuations. In order to ensure reliable vibration of the oscillator under all occurring operating conditions of the electromechanical vibratory element, complicated and expensive circuit arrangements and/or constructive measures are frequently required. In addition, the vibratory elements that should be interpreted as dipole network or quadrupole network transmission elements may have similar characteristics at different frequency points that occasionally may lie relatively far apart from one another. If this makes it possible to fulfill the vibratory requirements of the oscillator for different frequencies, the system oscillates rather randomly at one or the other frequency and thus makes it impossible to determine the filling state of the container. This means that expensive measures are also required here in order to ensure that the system always oscillates at the defined, desired frequency and reliable information concerning whether a certain level has been exceeded or not reached can be obtained.